1. Field of the Invention
The present invention relates to a toner for developing an electrostatic image. In particular, the present invention relates to a toner for developing an electrostatic image, the toner having superior low-temperature fixing properties and capable of forming an image of uniform density.
2. Description of Related Art
Electrophotographic imaging apparatuses require a toner for developing an electrostatic image (hereinafter may be referred to simply as “toner”) having superior low-temperature fixing properties in view of high image forming rate and energy saving for a reduction in environmental load.
In addition to an improvement in low-temperature fixing properties, such a toner is required to form high-quality images. In particular, a toner used for a paper sheet having an uneven thickness (e.g., a paper sheet composed of softwood fiber having a large diameter) is demanded to have superior physical properties for forming an image of improved quality because such a paper sheet may cause the uneven density of a toner image.
For example, Japanese Patent Application Laid-Open Publication No. 2015-64449 discloses a toner used for a paper sheet having an uneven thickness, the toner containing an amorphous polyester resin that forms ionic crosslinks by aluminum. This toner has pressure dependence. In detail, the toner exhibits low viscosity at a thick portion of a paper sheet because the thick portion comes into contact with a fixing member during fixation and receives a large amount of pressure from the fixing member, resulting in breakage of ionic crosslinks. In contrast, the toner maintains high viscosity at a thin portion of the paper sheet because the thin portion does not come into contact with the fixing member and receives a small amount of pressure, resulting in remaining ionic crosslinks. The high-viscosity toner remaining at the thin portion probably prevents the flow of the low-viscosity toner into the thin portion, resulting in small difference in amount of the toner between the thin and thick portions.
Unfortunately, the melting of the toner by heating during fixation has not been taken into consideration. The pressure-dependent toner hardly melts and forms a layer at the thin portion because the thin portion does not come into contact with the fixing member and receives a small amount of heat during fixation. Thus, the toner exhibits unsatisfactory low-temperature fixing properties.
The viscoelasticity of the toner disclosed in Japanese Patent Application Laid-Open Publication No. 2015-64449 is measured under conditions different from those under which the toner melts by heating during fixation. Thus, it is unclear whether the toner melts to form an image of uniform density.